首先，我赞同曹老师的观点：第一句中reform是抽象名词, 第二句中reform是可数名词。

网友说：两个句子结构差不多，一个the用在前，一个用在后，是否一样呢，所有格此类情况冠词使用是否有什么讲究?

of前面名词短语用不用the；of后面名词短语用不用the，咱们分开讨论：

第一句：He has urged reform of the welfare system.

of后面用the （welfare system），主要原因是system在句中是可数名词。

of前面reform没有带the, 薄冰先生有解释过。

   在当今英语里，其定语为of短语的不可数单形名词如表动作的话，其前用零冠词的情况就更多了，这大概是由于强调动作从而降低of短语的重要性的缘故。且看:

Construction of a modern theatre began this spring in western suburbs.

We know collapse of the lung does occur not only post-operatively but in other conditions as well.

It is appropriate that some time be devoted to thorough study of the results of the Apollo missions.

Omission of “the" in this case is not obligatory.

Clearance of this cheque could take a week.

Conversion of your heating system rom coal to gas will be costly.

…

第二句：He called for the reform of capital gains tax.

相信of前面the reform中的the不是你的疑问。那么of后面为什么不用the capital gains tax？

capital gains tax是税种的一类，这个短语在《牛津》《朗文》《麦克米伦》中以词条形式出现，并归为不可数名词词组，词典例句多以不带the的形式出现：

pay capital gains tax   

to reduce capital gains tax.

your liability to capital gains tax

所以用capital gains tax而不用the capital gains tax就不难理解了。